The present invention relates to a device for measuring and adjusting the out-of-track distances of helicopter rotor blades.
The invention also relates to a process for measuring and adjusting the out-of-track distances of helicopter rotor blades by using the aforesaid device.
In order to avoid any abnormal vibration of helicopter blades, it is important that the tips of the blades all rotate in the same plane. The track followed by the blades must therefore be precisely set.
The track can be coarsely adjusted as follows: marks of different colors are placed on the blade tips by means of chalks or felt pencils. The blades are turned and a piece of cloth (flag) is brought close to their tips. When the tips touch the cloth, traces of the said marks are transferred to the cloth providing a means of identifying any out-of-track distance. The track is adjusted from these distances by taking action on the respective lengths of the blade pitch control linkage.
This method has the drawbacks of lacking precision and of only being possible on the ground when there is little wind.
A more precise adjustment of blade track can be obtained by the following stroboscopic method: a stroboscopic lamp is held by an operator placed in the helicopter. The lamp is oriented in a fixed direction such that the blade tips cut the line along which the lamp aims and the lamp emits a ray of light whenever a blade tip passes through the line of aim of the lamp. Emission of the ray of light is controlled by a magnetic sensor that delivers electric signals by means of magnetic knife blades placed on a disk that is fitted to rotate on the rotor. The angular deviation between the knife blades is the same as that of the lift-blades, so that the position of the magnetic knife blades corresponds to that of the rotor blades. Moreover, each blade tip carries a reflecting target which is illuminated whenever it crosses through the ray of light emitted by the stroboscopic lamp, thus enabling the operator to distinguish the target. In order to distinguish between the targets, the target of each blade is of a different shape from that of the other blades. Given that the blades move at a speed greater than the perceptive ability of the eye of the operator, the operator distinguishes all the targets at the same time and can thus assess their relative positions which show the out-of-track distance of any blade in relation to the others.
The above stroboscopic method has the enormous advantage over the method described and based on a piece of cloth, in that it allows the blade tracks to be checked while the helicopter is flying.
Nevertheless, the method has a few drawbacks. Assessment of the relative position of the reflecting targets is in fact a delicate operation because the targets appear simultaneously in a very limited field of vision so that they are more or less superimposed on each other. Moreover, the need to give the targets distinct shapes is not conducive to precision of measurement so that the settings to which the measurements lead are not always very rigorously precise.
Furthermore, the greater the number of blades, the greater the difficulty for the operator to differentiate between the targets and of judging their relative positions. When a helicopter has six blades, the adjustment of blade track is therefore a very delicate operation. The same applies when a helicopter has less than six blades and when its design demands very low tolerances for blade track deviation distances.